Preparation, characterisation, and electrical properties of (K0.5Na0.5)NbO3 lead-free piezoelectric ceramics

Na0.5 K0.5NbO3 (KNN) ceramics were sintered at different temperatures (970 °C, 1000 °C, 1030 °C, 1060 °C, and 1090 °C) for 3 h by a pressureless sintering method. The powders had been synthesised by sol–gel method, using citric acid as a coordination agent and ethylene glycol as an esterifying agent. The effects of temperature on the phase, microstructure, dielectric, ferroelectric, and piezoelectric properties of the as-prepared ceramics were analysed. The results revealed that all of the ceramics had a pure perovskite phase with orthorhombic symmetry. The piezoelectric constant (d33), the relative dielectric constant (εr), the planar electromechanical coupling coefficient (Kp), and the remnant polarization (Pr) initially increased and then decreased with increasing of temperature in such KNN ceramics. The volatilization of sodium and potassium increased with increasing sintering temperature. Over the range of temperatures studied, those ceramics sintered at 1060 °C had the following optimal properties: (ρ = 3.97 g/cm3, d33 = 119 pC/N, εr = 362.46, tan δ = 0.05, Kp = 0.23, Pr = 11.97 μC/cm2, Ec = 10.35 kV/cm, and Tc = 408 °C).